The present application claims priority to Japanese Patent Application No. 2000-182612 filed Jun. 19, 2000, the disclosure of which is incorporated by reference herein in its entirety.
1. Field of the Invention
The present invention relates to a self humidifying type fuel cell system. More specifically, the invention pertains to an ion-exchange-membrane-equipped solid polymer fuel cell having a cation exchange membrane and an anion exchange membrane.
2. Description of the Related Art
There are several kinds of fuel cells. Among them, a solid polymer type fuel cell is compact and is capable of generating electricity even at a relatively low temperature.
The body of a solid polymer type fuel cell is a stack having a plurality of cells stacked over each other. As illustrated in FIG. 4, unit cells in the stack are connected and stacked in series through an interconnector 9 which is electrically conductive and gas impermeable plate.
As an ion exchange membrane 11 constituting the main part of the solid polymer fuel cell, a cation exchange membrane called a xe2x80x9cproton exchange membranexe2x80x9d is generally used. In the unit cell which is a basic component of a fuel cell, catalysts 7,8 are held on both sides of this proton exchange membrane 11 (proton conductive polymer film) and a gas diffusion layer 3 is disposed on the outside of each of the catalysts. This diffusion layer 3 is usually made of carbon paper and it serves as a collector layer which allows to pass therethrough electrons generated on the catalysts or a carbon-made electrode (hydrogen electrode oxygen electrode). As the above-described catalyst, metal catalysts such as platinum are used. Catalyst layers 7,8 are formed, for example, by having such a metal catalyst born on carbon particles and then mixing it with an electrically conductive polymer serving as a binder.
On the outside of each of the above-described unit cells, there exists a plate having a gas channel disposed therein and it is called xe2x80x9cinterconnectorxe2x80x9d 9. A reactant gas is fed from gas channels 4,5 disposed in the interconnector 9 and the electricity-generating capacity of the cell is determined by a fuel gas and an oxidizing gas to be fed. There are two gas flow channels, one is the channel 4 for feeding and discharging a fuel gas therethrough and the other one is the channel 5 for feeding and discharging an oxidizing gas therethrough. Accordingly, the interconnector has, on one side thereof, a face having the gas channel for feeding a fuel gas and, on the other side, a face having the gas channel for feeding an oxidizing gas.
The interconnector 9 is a current-carrying substance equipped with a function of separating a fuel gas and an oxidizing gas as illustrated in FIG. 4. The term xe2x80x9cfuel gasxe2x80x9d as used herein means a gas usually composed of a hydrogen gas and water vapor. The catalyst layer which is in contact with this hydrogen-gas-containing fuel gas is called xe2x80x9chydrogen electrodexe2x80x9d. The term xe2x80x9coxidizing gasxe2x80x9d as used herein means an oxygen containing gas such as air and the catalyst layer which is in contact with the oxidizing gas is called xe2x80x9coxygen electrodexe2x80x9d.
When this solid electrolyte fuel cell is connected with an external load, a hydrogen gas is decomposed into protons and electrons at the hydrogen electrode and transfers through an electrolytic membrane toward the side of the oxygen electrode. At this time, protons transfer in clusters, together with water molecules near these protons. This is so called electroendosmosis of water. By this phenomenon, the hydrogen electrode side of the membrane dries out.
In order to overcome the above-described problem, the hydrogen gas at the hydrogen electrode is usually fed in the externally humidified form, which suppresses an increase in the electric resistance due to drying of the membrane.
In order to attain a sufficient water vapor pressure, it is necessary to heat the hydrogen gas. It is however known that an excessive temperature increase heightens the water vapor partial pressure of a hydrogen/water vapor gas mixture, which exerts a bad influence on the supply of a hydrogen gas itself necessary for the reaction.
With the foregoing in view, various membranes for a fuel cell have been studied.
In a fuel cell, protons introduced into the side of the oxygen electrode react with oxygen on the electrode and generate water. This product water must be discharged outside promptly, together with water generated by electroendosmosis, because excess water clogs therewith the gas diffusion channel of the adjacent diffusion layer, thereby decreasing an effective electrode area. These waters are diffused toward the hydrogen electrode side in accordance with a moisture concentration gradient provided by the hydrogen electrode side. This phenomenon is generally called xe2x80x9cback diffusionxe2x80x9d.
It is the common practice to promote the back diffusion of water by decreasing the thickness of the membrane, thereby raising this concentration gradient. This method makes it possible to relax humidifying conditions while reducing humidification as much as possible.
When the membrane is thinned, however, about 1% of a hydrogen gas physically passes through the membrane, resulting in a problem such as a decrease in an electromotive force.
Various methods have been attempted for internal humidification. The first method is to obtain a necessary moisture amount by dispersing, in a membrane, a predetermined amount of a catalyst similar to that dispersed on the surface of the electrode catalyst, trapping hydrogen and oxygen which pass through the catalyst-dispersed membrane and generating water inside of the fuel cell. This method also involves a problem in lowering of the electromotive force. In addition, this method presumably causes lowering in the performance owing to a deterioration of the membrane caused by pin holes formed inside of the membrane by the heat upon reaction.
The second method is to mechanically trap moisture at the outlet of the stack and circulating it to the humidifying portion of the upstream stage of the stack. The term xe2x80x9cstackxe2x80x9d is a unit module having a plurality of unit fuel cells stacked one after another. This method is however accompanied with the problem that the mechanism of this method which traps water at the downstream stage of the stack inevitably enlarges the apparatus and makes it unsuitable as a mobile fuel cell.
The third method is to dispose a cooling passage on the side opposite to the hydrogen channel of the interconnector, and diffuse water to the interconnector itself, thereby humidifying the hydrogen on the opposite side. A carbon material is ordinarily employed for the interconnector, but the interconnector without any treatment allows a hydrogen gas to penetrate therethrough. Its gas tightness is therefore heightened by immersing it in a polymer such as phenol resin. It is however not easy to produce an interconnector which has, in good balance, improved hydrogen-gas tightness and capacity of diffusing water, thereby humidifying hydrogen.
With the problems in view, the present inventors have carried out an extensive investigation with a view to developing a self-humidifying type fuel cell system which, without using auxiliary equipment such as humidifier, can feed a reactant gas with water while controlling excessive increase of water.
As a result, it has been found that the above-described problems can be overcome by employing a self humidifying type fuel cell system wherein internal circulation of water is carried out using an anion exchange membrane MEA (membrane electrode assembly) and a cation exchange membrane MEA disposed alternately, and electroendosmotic water and product water are used while being circulated by the transfer of ions in the directions opposite to these membranes.
The present invention has been completed based on such a view.
An object of the present invention is to provide a solid polymer fuel cell system having a plurality of unit cells stacked one after another, each unit cell comprising an electrode of an anion exchange membrane and an electrode of a cation exchange membrane disposed adjacent but not in contact with each other, gas diffusion layers commonly disposed on both sides of these electrodes for allowing electrons generated on the catalysts to pass, and interconnectors which are disposed outside the gas diffusion layers and serve as a current carrier having a gas channel.
In the fuel cell system of the present invention, the gas channel for feeding a gas inside of the unit cell is disposed to connect the portion of the gas diffusion layer contiguous to the anion exchange membrane with the portion of the gas diffusion layer contiguous to the cation exchange membrane.
The present invention makes it possible to save auxiliary equipment such as humidifier, leading to simplification of the system due to a decrease in the number of parts or apparatuses, cost reduction and saving of the space.
In the fuel cell system of the present invention, an externally introduced gas is not affected by water vapor partial pressure increased by humidification. In addition, by the use of a cation exchange membrane and an anion exchange membrane, the water generation side and electroendosmotic side is reversed so that moisture necessary for reaction is available by the humidification at the upstream stage. Moreover, owing to the transference difference between OHxe2x88x92 ions and H+ ions, properties of the membrane can be changed by a difference in the area or thickness of the membrane.
Excess or shortage of humidity can be grasped by measuring the output voltage. This fuel cell system can be operated without any external humidification means under appropriate operating conditions. A circulation ratio of water can be adjusted by a flow rate or temperature of mainly air within a range not causing a flooding phenomenon which prevents smooth gas flow by excess water.
The present invention will hereinafter be described more specifically by embodiments. It should however be borne in mind that the scope of the present invention will not be limited to or by them.